Mononuclear aromatic polyisocyanates are well known and are widely used in the preparation of polyurethane and polyurethane/urea elastomers. These mononuclear aromatic diisocyanates generally include compositions such as toluenediisocyanate, phenylenediisocyanate, chlorotoluenediisocyanate and the like. ln the preparation of polyurethane and polyurea elastomers, the aromatic diisocyanates are reacted with a long chain polyol to produce a prepolymer which then may be further reacted with a short chain polyol or aromatic diamine to form a polyurethane or polyurethane/urea elastomer. Long chain polyols, e.g. those having a molecular weight of above 700, are generally utilized to form the prepolymer and the chain extender usually as stated, is a short chain polyol, e.g., C.sub.2 -C.sub.10 glycol, or an aromatic diamine. The long chain polyol provides flexibility and elastomeric properties to the resin, while the short chain polyol or aromatic diamine provides short chain segments to cross-link and add toughness and rigidity to the resulting resin.
A major problem with mononuclear aromatic diisocyanates is that they are toxic and because of their low molecular weight, tend to be quite volatile. Because of their toxicity and volatility, extreme care must be taken in the work place to avoid inhalation and damage to the respiratory tract and contact with the skin. There is some literature on the subject of reacting aromatic diisocyanates, particularly toluene diisocyanate, with short chain polyols to increase their molecular weight and reduce volatility and the utilization of these polyol extended aromatic diisocyanates in the formation of polyurethanes. Representative literature is noted in the following articles.
U.S. Pat. No. 2,969,386, German Pat. Nos. 756,058 and 870,400 show a method of producing organic polyisocyanates, and particularly urethane polyisocyanates, by reacting an aromatic diisocyanate with a polyol, such as ethylene glycol. However, as the '386 patent noted, there was a problem in that side reactions occurred and lead to the formation of polymeric products which contained free unreacted diisocyanate; in other words the process produced a mixture of compounds. The '386 patent attempted to solve the problem associated with the '058 and '400 patents by producing urethane polyisocyanates having free isocyanate contents of 13-18% but substantially devoid of unreacted diisocyanate. Such result was achieved by carrying out the following steps (a) reacting the organic diisocyanate with polyol in an amount in excess over that stoichiometrically required, e.g., greater than a 2:1 molar ratio at a temperature above the melting point of the mixture but low enough, i.e., below 100.degree. C., to avoid undesirable polymerization reactions; b) extracting the resin from the reaction mixture by contacting it with an aliphatic or acyclic hydrocarbon and c) separating the solvent layer which contains unreacted diisocyanate and polyol. By this technique one was able to obtain higher molecular weight urethane linked aromatic diisocyanates from the product layer.
U.S. Pat. No. 3,285,981 discloses reaction products of toluenediisocyanate and 2,3-butanediol, with the mole ratio of toluenediisocyanate to butanediol being at least 2:1, i.e., an NCO to OH ratio of at least 2:1 preferably 2-4:1. The product as prepared by dissolving toluenediisocyanate in a suitable solvent such as hexane and then adding butanediol to the solution. A solid white powder having a free NCO content of about 16% precipitated. The precipitate then was reacted with a milleable gum of polypropylene glycol, butanediol and toluenediisocyanate.
U.S. Pat. No. 3,218,348 discloses the preparation of a urethane polyisocyanate by the sequential reaction of a polyisocyanate e.g., toluenediisocyanate with a triol and then a diol. The patentees noted that when the procedure set forth in the '696 patent dsscribed above was carried out adding the diol and, then the triol or simultaneously adding the diol or triol to the isocyanate that the reaction product was unstable and would crystallize within a matter of minutes or days.
U.S. Pat. No. 3,076,770 discloses a Process for producing polyurethane foams by the reaction of an organic polyisocyanate with short chain polyols, utilizing the sequential addition of a triol and diol as in U.S. Pat. No. 3,218,348. The cellular polyurethane was prepared by removing free isocyanate from the reaction product and then reacting that with a polyester polyol to form the polyurethane.
U.S. Pat. No. 3,020,249 discloses a process for preparing polyurethane foams from an alkyd polyester resin and a reaction product of toluenediisocyanate and 1,2,6-hexanetriol. The hexanetriol-TDI adduct was formed by reacting toluenediisocyanate with the hexanetriol at temperatures of 100.degree.-120.degree. C., the toluenediisocyanate being included in substantial excess.
U.S. Pat. No. 3,023,228 discloses a process for producing urethane polyisocyanate urea systems by reacting a diisocyanate e.g., toluenediisocyanate with a short chain diol e.g. butanediol in the presence of a solvent, e.g., acetonitrile. Temperatures from about 10.degree.-35.degree. C. are suggested as being suited for forming the reaction product. Examples show reacting toluenediisocyanate with ethylene glycol or diethylene glycol in the presence of water and acetone for one to two hours at which time the reaction mixture solidifies and a crystal magma forms. The resulting low molecular weight products are valuable as reactants in the production of polyurethane plastics.